Process for coating metal



82 1959 w. F; LOUGHMAN 2,888,391

' PROCESS. FOR comma METAL Filed July 1, 1957 WASH/N6 APP! lC/l 7704 of Mrs/s7 can TIA/6 EXPOSURE WASH/N6 54 5671901. 9 7/0 STR/P'P/N;

WASH/N6 INVENTOR lfillz'az zzfi'loqy/Zman BY m ATTORNEYS United States Patent PROCESS FOR COATING METAL William F. Loughmau, Somerville, Mass.

Application July 1, 1957, Serial No. 669,206

8 Claims. (Cl. 204143) produce a surface which has resistance qualities far suods,. a surface which stands up against abrasion for at least 1500 cycles on a Taber abraser, as compared to the 175 cycle treatment to which standard surfaces are subjected, a surface that will'stand 1000 gram load with a C-5l7 wheel as compared to standard surfaces that withstand 6 to 16 gram load. The surface prepared according to the new method is not affected by detergents, alkalies or gasoline derivatives, nor do dilute acids affect it. The surface is also relatively insensitive to salt spray as compared to surfaces produced by standard techniques. An important feature of the coated metals prepared by the new method is that the fatigue element appears to be decreased rather than increased by the treatment.

By varying the composition of the coating applied to the metal to be treated and by application of known techniques, designs may be etched in the surface coating. The resulting designs will have practically the same resistance qualities as those heretofore described. For example, printing plates for offset can bemade which provide exceptionally sharp type in good half tone. Print-' ing plates so prepared need not be regrained and may be used to run hundreds of thousands of copies. Identification plates may be produced for fingerprints, photographs, handwriting, etc., in detail on stock .003 gauge which cannot be counterfeited or altered without. destruction of the plate. Fine detail maybe obtained and close toleranceskept up to 200'line screen half tone or any line or design which can be produced from a film on gauges from .003 to plate size. The high resistance to corrosion of the products prepared in accordance with the new method make them particularly suitable for use as cemetery markers or the like.

Another novel feature of the invention is the fact that the process maybe so carried out as to control the color of the metal underlying the coating with the result 2,888,391 Patented May 26, 1959 "ice etched sheet of metal, at which stage the desired color of the metal is obtained; and fourth, the application of a uniform layer of photosensitive resist or other coating to the treated metal, the metal being Wet when coating is applied. The subsequent steps of the process depend upon the desired end product and the type of resist applied to the metal but in every case include a heating step to harden the resist, treatment in an electrolytic bath and final washing and air drying of the finished plate.

For a better understanding of the new process reference may be had to the accompanying drawing, the single figure of which is a flow diagram of the preferred process.

A piece of chrome containing steel to be treated is first etched on one or both surfaces, depending upon the product to be obtained, in sulfuric acid of approximately 60 Baum. The carbon content onthe face of the metal is affected by the etching process and small pockets or cavities up to .002" in depth will be formed in the treated surface, the depth of the cavities depending upon the length of time that this step is continued. For example, a sheet 4 /2 inches surface area is preferably treated in the etching bath for a time period between 60 and 240 seconds, preferably about 2 minutes. The etched metal is then washed in hot water or steam, for example in a steam degreaser, to remove all acid.

The next step of the process, as indicated by the flow diagram, is one in which the metal surface is oxidized. This may be effected by heating the etched and washed metal plate in air to a temperature between 1400 F. to 1800 F. The heat treatment affects the color of the metal, the color changing as the temperature is raised. If the treated metal is to be utilized for producing a two-tone engraving, the temperature of the furnace is maintained at such value as to yield the desired background color for the final engraving. When the metal assumes this color, it is removed from the furnace and allowed to cool rapidly in air. The reason for the different color effects obtained in this step of the process is believed due to the formation of different thicknesses of oxide film on the metal which film acts like an interference filter to give color depending upon the thickness of the film.

Alternatively, and particularly if a black color is desired on the finished product, the oxidizing step may be performed by treatment of the metal in a salt bath heated to a temperature of approximately 260 F. One suitable composition of salt bath is:

According to the process, the bath is extended until short hair-like threads form about two to five mils in length.

Preferably, to a bath of the above type, a small amount, say 1%, of potassium hydroxide is added.

After the oxidizing step the metal is thoroughly washed in water. At this stage, the surface of the treated plate has a great affinity for water. The afiinity for water'is very apparent when a wet plate is suspended vertically. The moisture remains on the surface as globules instead of running off the surface. The exact cause of this condition of the treated plate is not positively known but probably the previous treatment comprising the etching followed either by heating in air or in the salt bath, has resulted in the formation of oxides on the surface of the plate that have great aflinity for moisture. In

order to get good results, the plate must have the afiinity for moisture at this point. At this stage the treated plate may, if desired, be stored for later treatment. Whether stored or not, the plate just before applications of the resist coating should be thoroughly Wetted in Water as maximum adherence of the coating can only be obtained when the surface of the plate is wet during application of the resist coating.

Suitable coatings are of two types-resists containing photosensitive components and enamel type resists. The further processing using the photosensitive type will first be described. The photsensitive resist may be of a more or less standard composition such as is used in the graphic arts provided there is incorporated therein some protein "containing material such as fish, animal, glue, egg albumin or the like. Preferably fish or animal glue is used. A formula that I have found very satisfactory is the "following:

Mix together:

Parts Ammonium dichromate 2 Chromic acid or potassium chromium sulfate 1 Water-ammonium solution 1 Water 6 to 8 Then to the above mixture add a mixture of protein as contained in animal, vegetable, fish or glue 12 parts, water 24 parts.

A coating of the above or other suitable mixture is then applied to one or both sides of the plate, as desired, as by dipping, brushing, spraying or the like. To insure a uniform coating, the coated plate, while the coating is still wet, is subjected to mechanical action, for example, to centrifugal force, or to a spinning action, the preferred type of action being determined in large part by the conformation of the article or plate.

if there is to be a design or legend on the finished product, the coated plate is exposed through a suitable transparency having the design or legend thereon, for example, a positive or negative film, to relatively intense light such as light from an arc lamp. The time of exposure need not be more than a few minutes. This hardens the resist where exposed to light. The exposed plate is then Washed off with Warm Water to remove the soluble unexposed portion of the resist and then the plate is heat hardened by subjection to a temperature of 400 to 650 F. This step of the process could be done by placing the exposed plate in a gas oven or by exposure to infra-red heat or in an induction furnace. Preferably when one side of the metal has been coated the heat is applied through the metal from the imcoated side in which case the uncoated side may reach temperatures of 200 to 300 F. above that of the coated side. This step forces the moisture out of the coating and provides a condition which makes the next step convert the protein to a hard polymer resin.

After heat hardening, the plate is treated in an electroiytic bath of sulfuric acid (40 to 60 Baum) and water. he plate is suspended on the anode and a lead shield is provided at the cathode. The bath draws ap proximately one ampere per square inch with a voltage across the plates of from four to six volts. The treatment in the electrolytic bath is carried on for a very few minutes, the time of treatment depending upon the area of the plate. During the electrolytic process metal is stripped from the part of the plate from which the resist has been removed, that is, from those areas which were not exposed to light. Upon removal from the electrolytic bath, the plate is washed and rubbed to re move any loose coating which may have remained over the unexposed areas. During the step, the above-mentioned conversion of the protein takes place.

The final product, assuming that the color of the plate after the oxidizing step was black, will be black over the areas exposed to light and will be substantially the color of the original steel, but with a dull finish, over the remaining portion of the plate.

If the entire coated area Was exposed to light, then the entire plate will be black; or whatever color resulted from the oxidizing step.

The resulting product after electrolytic stripping has a coating thereon which is essentially integral with the metal. Chemical action apparently occurs during the stripping which causes this complete union between coating and underlying metal. Thus, the final product differs from products of prior processes in that the resist coating applied during the process remains on the product as a permanent part thereof.

What is claimed is:

1. The process for treating alloy steels of the class consisting of austenite and ferrite having a minimum chrome content by weight of .11 to provide a product having an abrasive resistant and acid resistant permanent coating, which comprises etching the metal in a sulfuric acid bath to produce minute surface pockets therein, washing to remove the acid, oxidizing the washed metal and thereby changing the surface color of the metal, water Wetting the metal and applying a resist coating containing a protein to at least part of the oxidized metal before the metal is dry, subjecting the coated metal to a temperature of at least 400 F. immersing the metal in an electrolytic bath and finally passing electric current from the metal into the electrolytic bath to toughen the resist coating. I

2. The process according to claim 1 wherein the resist coating is photosensitive and wherein at least a portion of the coated surface is rendered insoluble by exposure to light and the resist coating is washed from any unexposed areas prior to subjection to heat treatment.

3. The process according to claim 2 wherein after coating and before exposure the coated metal is mechanically agitated to insure uniform thickness of coating.

4. The process according to claim 1 wherein oxidation is carried out in a heated salt bath which blackens the metal surface.

5. The process according to claim 1 wherein oxidation is carried out by heating in an oxidizing atmosphere, the resulting surface color depending upon the temperature to which the metal is raised.

6. The process according to claim 1 wherein, during treatment in the electrolytic bath the metal is suspended from the anode and a lead shield is provided at the cathode, the electrolytic bath comprising sulfuric acid of 50 to Baum.

7. The process according to claim 1 wherein the coated metal upon removal from the electrolytic bath is washed I and air dried.

in an electrolytic bath and finally passing electric current from the metal into the electrolytic bath to toughen the resist coating.

References fitted in the file of this patent UNITED STATES PATENTS Batcheller May 19, 1942 Batcheller May 8, 1945 

1. THE PROCESS FOR TREATING ALLOY STEELS OF THE CLASS CONSISTING OF AUSTENITE AND FERRITE HAVING A MINIMUM CHROME CONTENT BY WEIGHT OF .11 TO PROVIDE A PRODUCT HAVING AN ABRASIVE RESISTANT AND ACID RESISTANT PERMANENT COATING, WHICH COMPRISES ETCHING THE METAL IRR A SULFURIC ACID BATH TO PRODUCE MINUTE SURFACE POCKETS THEREIN, WASHING TO REMOVE THE ACID, OXIDIZING THE WASHED METAL AND THEREBY CHANGING THE SURFACE COLOR OF THE METAL, WATER WETTING THE METAL AND APPLYING A RESIST COATING CONTAINING A PROTEIN TO AT LEAST PART OF THE OXIDIZED METAL BEFORE 